1. Related Patent Applications U.S. Pat. No. 4,827,411, by A.H. Arrowood et al, issued May 2, 1989 and entitled "Method of Maintaining a Topology Database", discloses a method of maintaining a common database of link and node topology information at plural nodes of a computer network. Said method uses techniques disclosed in the present application.
Co-pending patent application Ser. No. 07/062,269, by A.E. Baratz et al, filed June 15, 1987, and entitled "Locating Resources in Computer Networks", discloses a technique for locating devices and other resources (programs, databases, etc) in computer networks which uses communication techniques disclosed herein.
Co-pending patent application Ser. No. 07/062,280 by A.E. Baratz et al., filed June 15, 1987, and entitled "Method For Disseminating Network State Information", discloses a technique for disseminating network state information in SNA networks, and which is related in general to this invention.
2. Field of the Invention
The present invention relates to computer networks and particularly to how to resynchronize the recovery of a logical full duplex session which is comprised of two physical half duplex sessions.
3. Terms and Concepts
The following discussion of terms and concepts may be helpful for understanding the problems solved by the present invention. Concepts and terms presently considered to be new are so indicated below. Others are described in one or more of these published references:
(1) Systems Network Architecture, Concepts and Products, IBM Publication GC30-3072-3, Copyright International Business Machines Corporation, October 1986; PA0 (2) Systems Network Architecture, Technical Overview, IBM Publication GC30-3073-2, Copyright International Business Machines Corporation, September 1986; PA0 (3) Systems Network Architecture, Transaction Programmer's Reference Manual for LU Type 6.2, IBM Publication GC30-3084-2, Copyright International Business Machines Corporation, November 1985; PA0 (4) SNA Format and Protocol Reference Manual: Architecture Logic For Type 2.1 Nodes, IBM Publication SC30-3422-0, Copyright International Business Machines Corporation, December 1986; PA0 (5) Systems Network Architecture Reference Summary, IBM Publication GA27-3136-6, Copyright International Business Machines Corporation, May 1985; PA0 (6) Synchronous Data Link Control Concepts, IBM Publication GA27-3093-3, Copyright International Business Machines Corporation, June 1986. PA0 (7) IBM System 36 Advanced Peer to Peer Networking (APPN) Guide, IBM Publication SC21-9471-0, Copyright International Business Machines Corporation, October, 1986.
In general, networks comprise communication links formed within physical media or electromagnetic wave channels, and equipment coupled to such links at "nodes" or points of connection. Computer networks are characterized by the presence of computers at some of the nodes, as distinguished from networks having only so-called "dumb terminals" at all nodes.
Nodes controlling communications in a network require certain information in respect to physical and logical characteristics and states of links and nodes ("topology" information), network error conditions, and resources available at the nodes ("resource" information). The term "network state" information is used presently to characterize information falling into any of these categories.
SNA networks presently classify nodes in four type categories--2.0, 2.1, 4, and 5--according to logical and/or physical characteristics of equipment contained at the nodes (e.g. see GC30-3073 above, pages 1-20 and 1-21). For present purposes, a new species of type 2.1 node termed type APPN is defined, and within that species two new subspecies of nodes types NN and EN are defined. Type APPN nodes as presently defined are nodes capable of certain advanced peer-to-peer networking (hence the acronym "APPN") types of communication operations to be described. Such operations involve transfers of node capability information and other network state information in certain new data link and session communication forms described later. Type EN nodes (also termed "end nodes") are situated at logical end boundaries of the network in that they do not act to communicate information between other nodes.
Type NN nodes (also termed "network nodes") are nodes which may carry communications between other nodes. End nodes generally contain end use devices such as printers, displays, etc., or computers such as data hosts which only support devices not accessible as network components. Network nodes contain computers which in addition to processing data are adapted for providing communication services for accessing resources at other nodes.
In such networks, information is communicated in various formats. Two forms presently of interest are data link type communications and sessions. In relation to sessions, data link communications are generally more direct (only between directly linked nodes), shorter and less extensive in the amount and type of information which can be transferred. Data link communications are more fully described in GA27-3093 above, and sessions are more fully described in GC30-3073 above. A "control point" is a node component which can activate sessions with other nodes and provides control services and information needed by other components of the respective node and/or control points nodes (see e.g. page 3-3 of GC30-3073 above APPN node has one and only one control point.
Of particular interest presently, are XID type data link communications (see Chapter 2 in GA27-3136 above) and sessions between network control points (see GC30-3072 above, page 23). XID communications are used for determining characteristics of other nodes. During activation, APPN nodes use XID communication to selectively instigate certain newly defined actions relative to other APPN nodes. Such actions include additional data link type exchanges, one to determine capabilities of the responding node and contingent on that determination a further exchange to establish a pair of oppositely directed half-duplex sessions of a newly defined relative to the responding node. These sessions together presently are termed a type APPN logical CP-CP session (or simply a CP-CP session), and they are used to permit the nodes to transfer additional network state information to each other as and when needed. Although each of the two sessions above is half-duplex, for reasons discussed later they are used only in simplex modes.
Logical units (LU's) manage the flow of information between end users and the network (refer e.g. to GC30-3073-2 above, at page 1-11). End users communicate with each other only indirectly via associated LU's. SNA architecture defines LU types 1, 2, 3, 4, 6.1 and 6.2 having specific responsibilities; types 1, 4, 6.1 and 6.2 supporting communications between programs. Associated with each LU type is a corresponding type of communication protocol.
4. Prior Art
Computer networks for processing and transmitting data are of course well known. In a typical such network, one or more computers running under control of respective operating systems, and application/transaction software, may interact through communications controllers and communications media to exchange data and to control operations of end use devices such as printers, displays, etc. Furthermore, computer networks susceptible of distributed control at plural active no are also well known,
Such networks usually are controlled in accordance with an architecture or set of basic protocols which ensures orderly flow of information between the nodes as well as between resources supported at the nodes. Examples of such architectures are given in: (1) "Computer Network Architecture", by S. Wecker in Computer, September 1979, and (2) "An Introduction To Network Architectures and Protocols", by P.E. Green in the IBM Systems Journal, Vol 18 No. 2, 1979. These articles describe various architectures, such as SNA, DNA, etc, in terms of a layered hierarchical structure where actions relative to accessing the physical communication medium are at the lowest layer or level, actions effecting exchanges with end users of the network are at the highest level, and other actions are at intermediate levels. In SNA, 7 such levels are defined in ascending order as: physical control level, data link control level, path control level, transmission control level, data flow control level, presentation services level and transaction services level. The functions performed at these levels are specified in GC30-3073-2 above at page 1-3. An architecture proposed by the International Organization For Standardization (ISO) comprises a 7-layer structure with levels having different but somewhat overlapping functions in relation to the SNA hierarchy. This is described by H. Zimmerman "OSI Reference Model--The Model Of Architecture For Open Systems Interconnection," IEEE Transactions On Communications, April 1980; and feature the layers: physical, data link, network, transport sessions, presentation and application.
According to the present invention a method is set forth for resynchronizing the recovery of a logical full duplex session, comprised of two half physical half duplex sessions, operating between two adjacent nodes.